The invention relates generally to an optical apparatus, and pertains more specifically to a system for producing an output beam having a preselected distribution of power and/or energy while minimizing undesired intensity variations at the output plane caused by sharp breaks between facets.
A laser device generally produces a beam of coherent light that has a wavefront of relatively small cross-section. In spite of the small cross-section and the coherency of the beam, the wavefront of a laser typically has a nonuniform power distribution that is stronger in the center than at the outer edges. The power variation may be between five and ten percent Furthermore, to make use of the beam, it is often necessary to expand the cross-sectional area of the beam, thereby spreading the non-uniformity across a larger wavefront.
When conventional lenses are used to expand the beam, the non-uniform power distribution of the wavefront is carried through to the expanded beam. In addition, the non-uniformity of the beam becomes more apparent as the wavefront is now expanded over a greater cross-sectional area. This non-uniformity is often detrimental to the performance of a system utilizing the beam as the system must be designed for some average level of beam power or another approach would be to somehow strip the beam of its lesser power outer portions, possibly through the use of an aperture. Neither of these alternatives enable optimum use of the beam""s power and it is very difficult to achieve a uniform power distribution, such as the plus or minus one percent variation that is often desired, by way of conventional lens systems.
Holographic elements have been created to function as conventional bulk optical elements. In these cases, the holographic element, whose orientations and spatial periods are correct for the purpose of diffracting the incident wavefront into a desired output location pattern, shape or image. However, when built to function as a basic lens, these holographic elements would also carry the nonuniform power distribution through to the output pattern, shape or image, thereby also inefficiently using the power of the optical source.
The problem of how to compensate for wavefronts having a nonuniform power distribution is addressed U.S. Pat. No. 4,547,037. In this patent discloses a multi-faceted holographic element which redistributes the light energy of an incident beam onto a second plane disclosed. This is accomplished by constructing each facet as an individual hologram or diffraction grating. Each facet is sized to be inversely proportional to the intensity of the portion of the beam incident thereupon to assuring that substantially the same amount of power passes through each facet. The light transmitted through each facet is diffracted to arrive at different locations on a second plane, relative to their locations in the holographic element. Each of the subholograms or diffraction gratings either expand or contract the portion of the incident beam passing therethrough to illuminate equal, but different, areas on the second plane, thereby producing an output beam at the second plane with a wavefront of nearly constant intensity.
A problem with devices incorporating the teachings of the ""037 patent is that if the power distribution of the incident beam upon the surface of the hologram deviates from the design parameters, then the power distribution of the output beam at the second plane will be similarly affected and thus no longer uniform. In optical systems, there are many causes for such deviation in the power distribution of the incident beam could occur. For example, power fluctuations due to the age of the components, or simply the replacement of the source due to failure. In addition, any misalignment within the system due to shock or age will produce an output wavefront having a non-uniform power distribution.
What is needed is an relatively inexpensive way to convert an incident optical beam having a wavefront with a non-uniform spatial energy distribution to an output beam having a substantially uniform spatial energy distribution that is relatively insensitive to fluctuations in positioning of the incident beam and spatial energy distributions within the incident beam.
Further, what is needed is a relatively inexpensive way to convert an incident optical beam having a wavefront with a non-uniform spatial energy distribution to an output beam having a preselected spatial energy distribution using a hologram that does not have regular breaks between facets in order to better minimize the intensity variations on the output plane caused by regular breaks between facets.
Further, what is needed is a relatively inexpensive way to convert an incident optical beam having an arbitrary wavefront to an output beam having preselected attributes, including preselected angular spread, such that the output beam is useful in photolithography. Photolithographic exposure systems are used to image the pattern of a mask onto a wafer for the purposes of exposing resist, or photoresist, on the wafer in a pre-determined pattern. Subsequent processing of the wafer results in the completion of layers that eventually form the desired device, such as an integrated circuit.
When the mask is used in a projection lithography system, such as a laser stepper with a 5:1 or 10:1 reduction ratio, the mask is often referred to as a reticle. The reticle or mask is typically formed by chrome regions on a transparent substrate. The chrome regions of the mask block the incident light, thereby imposing the pattern of the mask as an intensity variation on the light. In a 5xc3x97 laser stepper, the pattern of the reticle is reduced by a factor of 5 as imaged onto a wafer. Typically, in this application, the beam illuminating the diffractive is relatively uniform and has a rather narrow cone angle of divergence, i.e., limited spatial and angular energy distributions.
While masks and reticles control the intensity of light on the wafer, there is a need for an element that controls the angular distribution of the light on the wafer. By modifying the particular angular distribution of the light illuminating the wafer, one can extend the depth of the field and resolving power of photolithographic exposure systems. This element should ideally be inexpensive and relatively insensitive to fluctuations in positioning of the incident beam and to fluctuations in the spatial energy distributions of the incident beam.
Moreover, what is needed is a relatively inexpensive way to convert a collimated incident optical beam having a wavefront with non-uniform spatial energy distribution to an output beam having a preselected spatial energy distribution, or a preselected beam shape, that is relatively insensitive to fluctuations in positioning of the incident beam and spatial energy distributions within the incident beam. Additionally, what is needed is a relatively inexpensive way to convert an incident optical beam having a wavefront with non-uniform spatial energy distribution to an output beam having preselected attributes, such as spatial energy distribution, or a preselected beam shape, or a preselected angular energy distribution, that is relatively insensitive to fluctuations in positioning of the incident beam and spatial energy distributions within the incident beam.
The invention is a beam homogenizer for converting an incident beam having a non-uniform spatial energy distribution into an output beam of preselected spatial energy distribution. The incident beam is incident upon the beam homogenizer, formed as an array of facets where each facet is constructed to transmit any portion of the incident beam incident thereupon to an output plane spaced from the beam homogenizer so that the light transmitted through each of the facets overlap at the output plane to form the output beam which now has a substantially uniform spatial energy distribution.
Additionally, the invention is a beam homogenizer that minimizes undesired intensity variations at the output plane caused by sharp breaks between facets. At least part of a hologram comprising irregularly patterned diffractive fringes is illuminated by an input beam. That part transmits a portion of that beam onto an output plane, whereby the energy of the input beam is spatially redistributed at the output plane into a homogenized output beam having a preselected spatial energy distribution at the output plane such that the illuminated portion of the output plane is a predetermined shape and a predetermined magnitude.
Moreover, the invention is a beam homogenizer for converting a input beam having a non-uniform spatial energy distribution into an output beam having a preselected spatial energy distribution at an output plane while minimizing the intensity variation caused by breaks between facets. An input beam illuminates at least some of the facet areas of a hologram. The facet areas have irregularly patterned diffractive fringes. The facet areas transmit a beam such that at an output plane, the majority of the portion of the input beam transmitted through each of said illuminated facet areas overlaps the portion of the input beam transmitted through at least one other illuminated facet whereby the energy of the input beam is spatially redistributed at the output plane into a homogenized output beam having a preselected spatial energy distribution at the output plane. The array of facet areas is a computer-generated hologram, relatively insensitive to fluctuations in positioning of the input beam for incidence thereupon and to spatial energy distributions within the input beam. The homogenizer transmits the transmitted portion of the input beam at a preselected angular spread and illuminates a target area corresponding to a preselected spatial energy distribution desired at the output plane.
Additionally, the invention is a beam homogenizer system for converting an input beam having a non-uniform spatial energy distribution into an output beam having a preselected spatial energy distribution at an output plane while minimizing the intensity variation caused by breaks between sub-holograms. An input beam illuminates at least some of an array of computer generated sub-holograms whose size is determined independently of the intensity of the portion of the input beam incident thereupon, and being relatively insensitive to fluctuations in positioning of the input beam for incidence thereupon. Each sub-hologram diffracts a majority of the portion of the input beam incident thereupon so that at a target located at the second plane, the portion of the input beam diffracted by each of the illuminated sub-holograms overlaps the portion diffracted by at least one other illuminated computer generated sub-hologram to form an output beam. The intensity of the output beam is substantially equalized over a entire target. The output beam has a preselected angular spread and the target corresponds to a preselected spatial energy distribution desired at the output plane.
Additionally, the invention is a method of homogenizing an input beam having an arbitrary spatial energy distribution at a first plane into an output beam with a preselected spatial energy distribution at a second plane while minimizing the intensity variation caused by breaks between sub-holograms. Steps taken are providing a holographic optical element comprising an array of computer generated sub-holograms with irregularly patterned diffractive fringes, fixedly positioning the element at the first plane so that the input beam illuminates at least some of the sub-holograms, each illuminated sub-hologram expansively diffracting the portion of the input beam incident thereupon over an entire target at the second plane to superimpose the diffracted portions of all of the illuminated sub-holograms to form an output beam at the second plane, wherein the step of providing the holographic element comprises generating an array of sub-holograms that is relatively insensitive to fluctuations in positioning of an input beam for incidence on said array and to spatial energy distributions within the incident beam. In the invention, each illuminated sub-hologram expansively diffracts the portion of the input beam incident thereupon at a preselected angular spread and produces a preselected spatial energy distribution desired at the output plane.
Additionally, the invention comprises a beam homogenizer system for converting an incident beam having an arbitrary spatial energy distribution into an output beam having preselected spatial energy distribution at an output plane spaced from the homogenizer while minimizing the intensity variation caused by breaks between sub-holograms. An array of sub-holograms designed with an iterative encoding method such that each sub-hologram has irregularly shaped diffractive fringes, and such that portions of incident beam diffracted by several of said sub-holograms overlap at the output plane, whereby the output beam has a preselected spatial energy distribution that is relatively insensitive to fluctuations in positioning of an input beam for incidence on the homogenizer and to spatial energy distributions within the incident beam. Each sub-hologram transmits a beam with a preselected angular spread. The output beam has a preselected spatial energy distribution desired at the output plane.
Additionally, the invention is a beam homogenizer system for converting an incident beam having an arbitrary spatial energy distribution and limited angular energy distribution into an output beam having a preselected angular energy distribution while minimizing the intensity variation caused by breaks between sub-holograms. An array of sub-holograms, each of said sub-holograms having irregularly shaped diffractive fringes, and each of said sub-holograms containing pixels that exhibit phase skipping and the light diffracted by at least two of the sub-holograms overlap in the output plane to form an output beam. The output beam has a preselected angular spatial energy distribution that is relatively insensitive to fluctuations in positioning of an input beam for incidence on said homogenizer and spatial energy distributions within the incident beam. The output beam has a preselected spatial energy distribution and/or a preselected angular energy distribution.
Additionally, the invention is a beam homogenizer for converting an input beam of non-uniform spatial distribution into an output beam of a more-uniform distribution. A computer-generated hologram in the invention has a phase-transmittance pattern. The Fourier Transform of the phase-transmittance pattern is uniform over a preselected angular region. The pattern is made up of one or more binary phase elements.
Additionally, the invention is a system for modifying the angular spread of an incoherent or partially coherent beam of light. An incident beam propagating with a cone angle is diffracted by a diffractive diffusing element into a range of preselected angles, These angles are determined by or dictated by the cone angle of the incident beam and the Fourier Transform of the diffusing element.
Additionally, the invention is a photolithographic-optical system. An input beam illuminates a diffractive diffusing element. The diffractive diffusing element illuminates a mask by the element""s transmission of an output beam at a preselected angular distribution.
It is an object of this invention to convert an incident optical beam having a non-uniform spatial energy distribution to an output beam having uniform spatial energy distribution at an output plane.
It is a further object of this invention to convert an incident beam having a non-uniform spatial energy distribution into an output beam having a preselected spatial energy distribution at an output plane spaced from the homogenizer while minimizing the intensity variation caused by breaks between facets.
It is a further object of this invention to convert an incident beam having a non-uniform spatial energy distribution into an output beam having a preselected spatial energy distribution of a preselected shape at an output plane spaced from the homogenizer.
It is a feature of this invention that the optical beam having a non-uniform spatial energy distribution incident upon a homogenizer having an array of facets and the portion of the incident beam transmitted through each facet is imaged over an entire target on overlap at an output plane, thereby homogenizing the incident optical beam to produce an output beam of substantially uniform power distribution at the output plane. It is another feature of this invention that the homogenizer is a hologram and each of the facets are subholograms. It is yet another feature of this invention that the subholograms are designed to minimize interference effects at the output plane between the light transmitted through the facets.
It is a feature of this invention that the incident beam having a non-uniform spatial energy distribution is converted into an output beam having a preselected spatial energy distribution at an output plane spaced from the homogenizer while minimizing the intensity variation caused by breaks between facets. It is a further feature of this invention that an incident beam having a non-uniform spatial energy distribution is converted into an output beam having a preselected spatial energy distribution of a preselected shape at an output plane spaced from the homogenizer.
It is an advantage of this invention that the homogenizer may be developed by computer generation techniques and may be fabricated relatively inexpensively. It is another advantage of this invention that the homogenization is relatively insensitive to fluctuations in the power density of the incident beam. It is a further advantage of this invention that the intensity of the output beam is substantially insensitive to the location the incident beam falls on the homogenizer.
It is a further advantage of this invention that it can convert an incident beam having a non-uniform spatial energy distribution into an output beam having a preselected spatial energy distribution at an output plane spaced from the homogenizer while minimizing the intensity variation caused by breaks between facets. It is a still further advantage of this invention that the invention can convert an incident beam having a non-uniform spatial energy distribution into an output beam having a preselected spatial energy distribution of an arbitrary preselected shape at an output plane spaced from the homogenizer.
It is a still further advantage of this invention that it can convert an incident beam having arbitrary spatial energy distribution and limited angular energy distribution into an output beam of preselected angular energy distribution or of preselected shape at an output plane spaced from the homogenizer.